The rolling of high-strength materials to the smallest final thicknesses requires the smallest possible working roll diameters for technical reasons in terms of shaping as well as for economic reasons. The prior-art cluster mills have proved to be suitable for these applications. The existing and increasing demand for steel strips of the smallest possible final thickness was one of the reasons behind the development of high-speed 12- and 20-roll mills. The essential feature of these cluster mills is, besides the number of rolls, the direct hydraulic adjustment, with which even very narrow strip thickness tolerances can be guaranteed. Adjusting mechanisms exerting qualitatively different actions on the nip contour are available for controlling or regulating the surface evenness of the strip. The rolls and support axles with supporting rollers are arranged in two bearing housings which are vertically movable independently from one another. Their displacement makes it possible to compensate large amounts of roll wear and even different working roll diameters, which can be optimally used within narrow limits. The latter possibility, i.e., the use of different working roll diameters, as well as the relatively high roll speeds make cluster rolls appear attractive even for broad strip thickness ranges. Twenty-roll mills offer advantages in rolling thin, hard materials because of their small working roll diameter relative to their surface length.
Both 12-roll and 20-roll mills have two sets of rolls with working rolls at the ends of the two sets of rolls facing each other, which limit the nip and act on the material being rolled from two sides. With the exception of the working roll, each set of rolls has a number of inner intermediate rolls and outer supporting rollers, on which the working roll is supported. Direct adjustment in 20-roll mills makes it possible to optionally install working rolls with diameters at ratios of up to approx. 2.5, e.g., with diameters between 30 and 70 mm, without additional conversion measures and especially without changing the number of rolls.
The type of the roll mill, e.g., two-, four- or 20-roll mills, are to be selected corresponding to the rolling task, i.e., as a function of the strip width, the minimum and maximum strip thicknesses, the strip material, etc.
It has also been known in connection with rolling to the smallest possible final thicknesses that different types of mills, which fulfill different tasks, can be used in multistep rolling processes. It has been known, e.g., that it is possible to use cluster mills with 12 or 20 rolls and working rolls with small diameters to arrive at a very small final width of, e.g., 0.01 mm or 0.05 mm, and subsequently to subject the surface of the rolled stock to sizing by passing the rolled stock through a mill of a relatively large diameter without causing any substantial additional change in the final thickness reached with the cluster mill, in an economical manner, i.e., in the smallest possible number of passes, from a relatively great initial thickness.
The relationship between the number of rolls and the roll diameter can be explained most simply by thinner working rolls, i.e., working rolls with a smaller diameter showing a stronger tendency than thicker rolls to whip and by a better support of the working rolls being therefore necessary, i.e., in order to avoid the whipping, which requires the use of a larger number of supporting and intermediate rolls. This means that two- and four-roll mills are used for sizing with the thicker rolls.
The types of cluster mills used to date have a set number of rolls. A 12-roll mill always operates with 12 rolls, and a 20-roll mill always operates with 20 rolls. This means that two different mills must be used in the case of a first rolling process, in which a very small final thickness from a relatively thick starting material is reached in one pass or at most in a few roll passes and surface finishing is necessary. First, a cluster mill with, e.g., 20 rolls, of which the working rolls are very thin, is needed for reducing the thickness of the rolled stock, and a mill containing only a few rolls is needed, in which the working rolls have a relatively large diameter.